System of picture transmission



1940- D. K. LIPPINCOTT 2,225,43fi

SYSTEM OF PICTURE TRANSMISSION Filed Aug. 26,1932 2 Sheets-Sheet 1 1INVENTOR,

1940- D. K. LIPPINCOTT 2,225,436

SYSTEM OF PICTURE TRANSMISSION Filed Aug. 26, 1932 2 Sheets-Sheet 2 5 NHAMPLIFIER. 1L I INVENTOR,

55 1 v Q I 1 Patented Dec. 24, 1940 PATENT OFFICE 2,226,43 SYSTEM OFPICTURE TRANSMISSIO Donald K. Lippincott, Larkspur, Califi, assignor, bymesne assignments, to Farnsworth Television & Radio CorporatiomDover,Del., a corporation of Delaware Application August 26, 1932, Serial No.630,524

14 Claims.

My invention relates to systems of electrical picture transmission, suchas television, photo telegraphy, and the like, and particularly tosyscurrent which is passed through a resistor. The- 5 finitesize of theaperture causes what is known as aperture distortion, that is, .a suddenchange in illumination from point to point of the picture field isrepresented by a current which increases gradually during the timerequired by the aperture to pass across the change, so that sharp linesare inevitably blurred in the measure of the aperture width. A further,similarblurring occurs at the receiver, thus doubling the effect. One ofthe objects of this invention is to elim inate this aperturedistortionefiect at the transmitter, thus reducing the total distortionof this character to one-half.

In the usual method of transmission, the aver age or mean illuminationof the picture field is represented by a direct-current component. Thiscomponent is difiicult to transmit, particularly in radio transmissionsystems, and even in wire transmission systems it is usually avoided. Itis customary, therefore, merely to transmit 35 the fluctuations ofillumination around this mean density, the average level of illuminationbeing re-established at the receiver either arbitrarily or, in somecases, automatically by the use. of a separate channel. Another objectof my inven- 40 mean level of illumination, together with illumin'ationfluctuations from point to point, on the same channel and without theuse or complicated equipment.

In general, changes in illumination. over a picture field occurabruptly, and are truly represented by square-fronted waves or pulses,occurring at random, that is, without any defined periodicity. P. T.Farnsworth, in his co-pending application, Serial'No. 500,092, filedDecember 4,

1930, and entitled "System of pulse transmis-.

sion, and now United States Patent No. 2,026,379, issued Dec. 31, 1935,has shown that the picture may be reproduced if only a portion of 55 theFourier component frequencies of these single tion is to provide amethodof transmitting the pulses are transmitted, and a proper restoringnetwork is used at the receiver. That this may be accomplishedsatisfactorily, requires that the pulses be filtered to remove thehigh-frequency components, and that these filters be practically 5without phase distortion. Such filters are difficult to construct, andanother object of my invention is to provide a scanning system wherebyeither square-fronted waves, or waves resulting from the removal ofcertain definite frequencies 1 from such square-fronted waves, may begenerated Without the use of filters or other auxiliary apparatus.

Among the other objects of my invention are:

- to provide a scanning system which maybe ap- 5 plied with any type ofscanning mechanism, whether cathode ray, oscillating mirror, or scanningdisk, and with either beam scanning (the so-called flying spot) or imagescanning; to provide a scanning system wherein synchronizing pulses'areautomatically generated without the use of additional equipment; toprovide a system wherein the entire picture transmission is accomplishedon a single wave, the positive portions of the wave carrying thepicture, while the negative portions accomplish the synchronizing; andto provide a scanning system wherein variations' in scanning speed, overthe various portions of the picture field, do not result in departuresfrom the true illumination level as long as the transmitting andreceiving scanning apparatus travel at the same velocity.

Other objects of my invention will be apparcut or will be specificallypointed out in the description forming a part of this specification, butl I do not limit myself to the embodiment of my invention hereindescribed, as various forms may be adopted within, the scope of theclaims.

Referring to the drawings: V

Figure 1 is a simplifiedschematic diagram illustrating my invention asapplied to a cathode ray television transmitter.

Figure 2 is a transverse sectional view,.on a larger scale, of the anodeand shield used in the transmitter tube of Figu e 1. r Y

Figure 3 is a view of the anode shield, showing the elongated scanningaperture.

Figures 4, 59nd 6' are fragmentary views of anode shields, showingapertures of various types.

Figure ,7 is a graph indicating the wave form produced by the scanninggenerator or oscillator of Figure 1. j

Figure 8 is a schematic view of an idealized picture field, togetherwith graphs indicating the current and voltage waves produced byscanningsuch a field in accordance with this invention.

Figure 9 is a fragmentaryview of a pair of scanning disks, illustratingthe adaptation of this invention to mechanical scanning.

Figure 10 is a simplified graph of a wave form produced in a singlescanning cycle by the apparatus of Figure 1.

Figure 1-1 is a similar graph showing the wave form as produced by theapparatus oLFigure 9.

Figure 12 is a simplified schematic diagram showing receiving equipmentfor'use with this invention.

Scanning systems of picture transmission, as distinguished fromnon-scanning systems wherein separate channels are required for eachelementary area of the picture field, all operate in accordance with thesame basic principles,

although they vary widely in detail. Thus, all

use a photosensitive element which controls the signal currents, and alltraverse the picture field with an, aperture of elementary size in orderto limit the graduation of current at any instant to that produced bythe light from a particular elementary area of the pictured surface. Inthis statement, the term picture field is used in its broadest sense,since the actual position of the aperture; relative to the-object and tothe photosensitive element, differs greatly in different systems. Thusin the simplest form of the Nipkow disk, an image of the objecttobepictured is focused on the rim of the moving disk, which carries theaperture. developed form of the same scanning equipment, a beam of lightis projected throughlenses in the moving disk upon the object itself,and light the latter in this case not affecting the, light itfromthis'beam is reflected from the object to the photosensitive cell. Inthe Farnsworth cathode ray system, an electrical image is formed, whichis moved across the scanning aperture,

self, but, instead, the electron stream initiated by the light. The sameprinciple is even adhered to in the more primitive methods of picturetransmission in which photosensitive cells are not used, but wherein anelectrical-feeler or contact traverses conducting and non-conductingportions of a specially prepared picture or print. In each of thesecases, however, the fiux through the aperture, whether that fiux belight, free electrons, or electric current in the usual sense,

determines the current flowing, and the aper-- ture is moved in somemanner relative to the picture field.

Considered broadly, my system comprises the use of an aperture ofelementary width, but whose length is substantially equal to that of thepath it describes across the picture surface. The end of this apertureis traversed across the field, in the direction of the aperture length,to disclose a varying area of the field. The area embraced by theaperture is thus constantly changing, disclosure and occultation of theaperture occurring alternately and preferably at different rates.Whether the disclosure be gradual and the occultation sudden, or whetherthe reverse be the case, is purely a matter of choice.

, This produces a current which is at a minimum (usuallyzero) when theaperture is fully occulted, at a maximum .when maximum area of thepicture field is disclosed, and which, between these conditions, variesat a rate which depends upon the illumination of'the particular portionof the picture field which isbeing traversed by the advancing end of theaperture. The aperchange. I the voltage pulses produced are all of onesign,

In a more highlyv an inductance coil, across which it produces a voltagewhich is proportional to its 'rate of During the period of thedisclosure,

while during the period of occultation the voltage pulse or pulses willbe of opposite sign. If the disclosure is gradual and the occult'ationsudden, it is preferred to use the disclosure pulses as the picturepulses, which may arbitrarilybe denominatedpositive pulses, while thenegative'occultation pulse is used as a 1 synchronizing pulse. Ifoccultation is gradual' and disclosure sudden, the occultation pulses become positive pulses.

A rectifier is used at the receiving end, so that the illumination ofthe received picture field is proportional to the intensity of thepositive pulses, while the negative pulses, occurring between thescanning sweeps of the field, have no eilect upon the pictureillumination. The receiving aperture, it may be noted, is of the usualtype, since the scanning system of the present invention is one of thefew which are not directly reversible for transmission and reception.

If the advancing end of the aperture be rectangular, the voltagesgenerated across the in- 3 ductance coil will be true square-frontwaves, but by shaping the end of the aperture so that it represents a.curve of a desired wave form. that is, a wave having any particulardesired frequency distribution, the pulses generated by 3 the advance ofthe aperture across a discontinuity in illumination will have this waveform,

and hence it is unnecessary to provide a liltherefrom. 4

The/nature of the invention maybe understood more fully by reference tothe accompanying drawings. A preferred embodiment, as applied to thesystem of television-described in the above-mentioned patent of Philo T.Farnsworth, is shown in Figure 1. A cylindrical evacuated envelope I,usually of glass, has at one end a flat surface 2 upon which isdeposited a photoelectric cathode coating 4. At the opposite end of thetube is a plane window 5, immediately within which is mounted acylindrical shield 6, having a narrow elongated aperture l formed in theside facing the'photosensitive cathode.

Mounted within the shield is the anode or target 9, which is supportedat its ends by glass beads Ill. The shield 6 connects through a batteryor other potential source I I with the cath- Y ode 4. The anode 9 isconnected through an inductor l2 and a resistor M to an intermediatepoint on the battery Ii.

An image of the pictured area is projected by a suitable lens system onthe cathode 4, liberat-' ing electrons which are drawn forward to theanode by the potential of the battery H. These electrons are focused toforman electrical im- 6 age in the plane of theaperture by directcurrent flowing through the solenoid l5, which surrounds the tube I. Thedetails of this are not shown in the present drawings since they arefully disclosed in the above-mentioned Farns- 7 worth patent, and sincethey relate only indi- 4 cell circuit by means of a transformer.

of the slope or saw-tooth wave type, the wave form being substantiallythat shown in Figure 7. The flux generated by these currents in thecoils |6 deflects the electrical imageacross the aperture I, and in thedirection of its length. This deflection is so adjusted that whenmaximum current fiows in one direction in the deflecting coils the imageis removed entirely from the aperture, that is, the aperture is fullyocculted as regards the image. At the peak of current in the otherdirection the aperture crosses the entire width of the image, receivingelectrons from an area thereof which corresponds to the entire width ofthe picture field but of elementary height. it

In the preferred method of using this equipment, the electrons from theimage, which enter the aperture 1, liberate secondary electrons from thetarget 9 by their impact, these secondary electronsbein'g drawnback tothe shield 6, owing to its higher potential. This causes current to flowthrough the-resistor I4 and inductor l2, causing voltage drops therein.

A vacuum tube 28 is connected across the inductor |2,'-and a second tube2i across the re sistor M. The cathodes 22 and 24 of these tubes bothconnect to the junction between the resistor and the inductor, the grid25 of tube 20 connecting at the opposite end of the inductor, while thegrid 26 of tube 2i connects to a variable contact 21 on the resistor 24.

. The plates 30 and 3|, of the two tubes are connected in parallel to aresistor 32, which in turn is connected through a voltage source 34 tothe cathodes and ground. The output leads 35 and 36 may connect to anysuitable amplifier or line along which it is desired to transmit theresulting signal.

The object of this arrangement of vacuum tubes is to neutralize orcancel out the effect of any. voltage drop which may exist in the coill2 due to its resistance, for since the elements I2 and M are in series,a proportional voltage drop will occur in the latter element, the dropsin the two elements being impressed upon the grids of the tubes inopposite phase. The contact 21 is adjusted so that, with a constantcurrent fiowment, the current in the resistor 32 will be directlyproportional to the 'voltage due to the inductance only of the coil l2.

A similar cancellation of the resistive drop due to the picture currentcould, of course, be accomplished by coupling the tube 20 to the photo-Owing to the difficulty in eliminating phase and frequency distortiondue to leakage reactancein a transformer, however, the arrangement shownis preferred.

The operation of the system is illustrated in Figure 8, wherein therectangle 40 represents a tion of the portion of the field which isbeing traversed by the leading edge of the aperture, The resultingcurrent is shown by the curve of' Figure 8, starting at zero where theaperture first starts to traverse the field, and climbing at area 44,which is assumed to have zero illumina- I tion. h

The inductive voltages produced by this current are proportional, not toits intensity, butto its rate of change, or, expressed mathematically,

E=L di/dt. This voltage is shown by the curve 46 of Figure 8, the dottedportions of both curve 45 and curve 46 indicating potions of the curvenot yet traversed by the advancing aperture. It will be seen that thesechanges are abrupt, and that aperture distortion effect in the directionof the motion of the aperture has been eliminated.

At the end of the scanning sweep the aperture traverses the field in theopposite direction, the

current decreases continuously, and the voltages produced across theinductance coil l2 are reversed both in time and in sense. Thiscondition is shown in Figure 10, which represents the voltages producedby scanning a field having but two variations in illumination. Thefirst, or greater illumination, is indicated on the scanning half of thecycle by theportion 50 of the curve, while the second or lesserillumination is shown by the portion 5|. The return sweep, where ascanning wave of the type shown in Figure 7 is used, occurs with muchgreater speed. As a result, a more intense negative voltage 5| isproduced while the end of the aperture scans the portion of the fieldrepresented on the direct sweep by the portion 5|, and a still moreintense portion 50' occurs while the end of the curve the same is trueof the portions 5| and 5|. The

wave may therefore be transmitted as a true alternating-current wave,whose zero axis indicates zero illumination.

It is a mathematical characteristic of waves of this character, bothsides of which carry equal energies, that they contain no frequencieslower than their own period, that is, the line frequency, whereas thewaves generated by the ordinary elementary aperture contain allfrequencies down to zero.

A receiver for this wave is shown schematically in Figure 12. Theincoming leads 35 and 36' may be the leads 35 and 36 of Fig. ,1, or anyother carrying an equivalent signal. These leads are connected to anamplifier 55, whose "output terminals connect respectively to the grid56 and cathode 51 of an oscillight or cathode ray receiving tube 58.

The anode 59 of this tube connects through a battery or other source ofpotential 60 to the cathode, and serves to generate therewith a beam ofcathode rays which may be focused in any accepted manner onthefluorescent screen 6| formed on the end of the tube 58. This beam isdeflected by currents flowing in the coil 62," and generated by anoscillator 54, which produces.

currents of the same wave form as those produced at the transmitter bythe oscillator IT. 'The oscillator 64 is, however, provided with input'terminals which are connected between the grid 56 of the tube 58 andits cathode 51.

We will first consider the case wherethe grid 56 is so biased that themaximum negative pulse is required toreduce the illumination, producedscanning spot. Only about one-fifth of the time would. be required forthis return sweep as for the effective sweep, but its effect would befive times as intense, and the eye would therefore integrate the tworesults to produce complete cancellation. The same would also be true ofthe portions 53 and of the curve, since the effect produced at thetransmitter will always be proportional tothe product of light intensityand scanning speed. It is to be noted that this, is a valuable feature,since, with the transmitter and the receiver in step, variations inintensity due to varying speed of scanning will always cancel out.

The grid 56 is, however, biased through a resistor 65 by a battery orother potential source 66, and this bias is so adjusted that the spot isreduced to zero for zero level of the incoming signal. For this reason,although the positive portion of the incoming wave can produce increasesin illumination, the negative portion of the wave is without effect,since in this instance negative illumination has no meaning.

One method of adjusting this bias on a received signal is merely toadjust the picture field for maximum contrast. If the bias be. too farpositive, the negative half of the incoming wave will cancel outportions of the positive half, tending to reduce the picture to a commonmean level of illumination. If the bias be too far negative, thepositive portion of the wave cannot produce its full effect, andtherefore the contrast will be decreased. For the properadjustment,contrast will be a maximum, and at the same time the true pictureeffect, as relates to the average illumination of the picture comparedwith its variations in illumination, will be preserved.

It will be noted that this condition'corre sponds to a completerectification of the picture wave. The positive half of the waveproduces ceiving oscillight the picture, while the negative half has noefiect thereon. A separate rectifier 'may be used to accomplish this ifdesired, but the use of the re itself as the rectifier is to bepreferred.

The negative half of the wave is not, however, without its use. I Thecontrol terminal of the oscillat'or 64 is connected directly to the gridof the oscillight, and the negative pulses will therefore hold theoscillator in step, without the intervention of any auxiliary apparatusfor this purpose. Furthermore, no portion of the picture wave can swingas far negative as the reverse half of the-wave always swings, andtherefore there is no tendency on the part of the oscillator 64 to pullinto step with the dark portions of the.

picture. n w

It will further be noted that it is unnecessary, in case the picturewave is modulated upon a entire negative half of the wave. Themodulating equipment may be so set that the .zero axis of the curve of.Fig. represents perhaps 25 or 34% of the mean carrier amplitude. Thenegative portion of the wave of Figure 10 would therefore produce fargreater than one hundred percent modulation, but since this can merelyreduce the carrier to zero, and not beyond, the full synchronizingeffect will be obtained, while all of the efiective picture amplitudewill be transmitted.

Thus far in the present discussion it has been assumed that the end ofthe scanning aperture 7 is rectangular, as shown in detail in Figure 6and indicated by the reference character 70. In his above-mentionedpatent, Philo T. Farnsworth has shown that the removal of high-frequencycomponents from a square-fronted wave produces av wave form which risesin a manner approximating a sine curve to a maximum, and then oscillateswith decreasing amplitude'about the by the inductance coil I2 to producea voltage Wave whose form is represented by the outline H of theaperture. There will thus be produced a wave whose form is that of asquare-front wave which has been passed through an ideal filter, withoutphasedistortion, and this result is achieved without the introduction ofany additional apparatus. The efiect may be modified by forming theaperture substantially as that shown by the reference character 12 inFigure 4. In this case the outline of the aperture is shaped inconformity with the wave form of a square-front wave wherein the cut-offis less abrupt than in the case of the aperture II, that is, thehighfrequency components are attenuated proportionately to frequency.

The use of an aperture of this character is advantageous in thatmultiple images are not produced when the wave is passed through arestoring network.

Figure 9 illustrates the application of my invention to disk scanning.In this figure only portions of the scanning disks are shown, since thismethod of scanning is well known and the purpose of the showing ismerely to indicate the generality of the present invention. The disk 75has the usual spirally arranged apertures 16,

but in place of being square or round, each of the apertures iselongated, being of. elementary width and of a length equal to thetransverse dimension of the picture field.

Aseparate disk 11 is mounted on an axis parallel to that 'of the disk75, and its circumference is provided with serrations 19, theseserrations being formed like saw teeth whose depth is equal to the widthof the apertures 16, and whose spacing is substantially equal to theother dimension of the picture. In the present instance, the rotation ofthe disk 15 is presumed to be counter-clockwise, while the rim of thedisk 11 moves down. It will be seen that the final or inner aperture ofthe spiral is about to be occulted suddenly by the advance of the tooth19'. The aperture 76' will next traverse the picture field, the portion80 of the advancing serratlon retreating before the aperture as itadvances until it, in' turn, is suddenly occulted by the advanceof thetooth 8L:

The'wave form produced vby this arrangement is shown in Figure 11, wherethe same field is supposed to be scanned as in the case of Figure 10.The advancing aperture 16' will trace out the portions 50 and SI of thecurve, but in this case the occultation is not performed in the samedirection as the scanning, and the negative half of the wave, althoughencompassing the same area as the positive half, will be a difierentshape, probably practically square as is show by the portion 82.

Reversal of the direction of rotation of the two disks will merelyreverse the wave, and have no further effect. In other words, in thiscase the disclosure will be sudden and the occultation graduahbut thewave form and the effect produced will be exactly the same as before. Inthis case, as before, the receiver will be adjusted to.

he scanning disk, and the disk ll eliminated. y

In this case, the two halves of the wave could be symmetrical, but thetime required to transmit each image would not be so economically used,I

It will be seen, moreover. that the ends of the aperture 16 may beshaped to produce a desired wave form. in exactly the same manner as arethe ends 01 the apertures 10 and it.

I claim:

l. The method of scanning an area to be electrically picturedwhichcomprises disclosing and then occulting successive complete lines acrosssaid area, said disclosure and occultation being at different rates. Y

2. In a method of picture transmission wherein a current flow isgraduated by traversing successive lines of, more than elementary lengthacross the picture field with an aperture, the step of alternatelyincreasing and decreasing the area of the field embraced by saidaperture, the rates of said increase and said decrease being materiallydifferent.

3. In a method of picture transmission wherein a current flow isgraduated by traversing the picture field with an aperture, the step ofalternately increasing and decreasing the area of the field embraced bysaid aperture to vary said current flow, inducing with the controlledcurrent a voltage having a wave whose sign is dependent on the sign ofthe change in said area, and utilizing one half of said wave toreconstruct the picture and the other half of said wave to synchronizethe transmitting and receiving operations,

4. In an electrical picture transmitter including a current sourcehaving a light actuated current-controlling element and a picturecircuit connected thereto, scanning means including i an elongatedaperture of a length 'suflicient to extend across the picture field andmeans for alternately disclosing and occulting portions of said field asrelated to said current-controlling element with said aperture, one ofthe disclosing and occulting operations of each cycle of operation beingeffected-by the traversal of an end 'of the aperture across said fieldin the direction of its length and the other of saidpperations in eachcycle being effected at a different speed,

and means responsive to. the rate of change of the current controlled bysaid element for impressing a signal on said picture circuit.

5. In combination, means for forming an electrical image of a field tobe pictured, a shield having an aperture therein substantially in theplane of said image, said aperture being of ele-i mentary width andsubstantially equal to the dimension of said image in length, a targetprotected by said shield positioned to receive the portion of saidelectrical image passing through" said aperture, and means fordefiectingsaid image across,said aperture in the direction of the lengththereof.

6. In combination, means for forming an electrical image,a shield havingan aperture therein substantially'in the plane of said image, saidaperture being of elementary width and substantially equal to thedimension of said image in length, a target protected by said shieldpositioned to receive the portion of said electrical image passingthrough said aperture, means for deflecting said image across saidaperture in the direction of the length thereof, an inductance coilconnected in series with said target, and an amplifying tube connectedacross said coil.

7. In combination, means for forming an electrical image, a shieldhaving an aperture therein substantially in the plane of said image,said aperture being of elementary width and substantially equal to thedimension of said image in length, a target protected by said shieldpositioned to receive the portion of said electrical image passingthrough said aperture, means for deflecting said image across saidaperture in the direction of the length thereof, an inductance coilconnected in series with said target, an amplifying tube connectedacross said coil, and means for neutralizing theeffect of the voltagedrop due to the resistance of said coil.

8. In combination with a scanning system comprising means forintegrating a current flow in accordance with the illumination of alinear area of a picture field, aninductance coil connected to pass saidcurrent, a resistor connected in series with said coil, and a pair ofvacuumtubes having control electrodes connected across said coil andsaid resistor respectively and in opposed relation so that resistivedrops due to said current flow will produce opposite effects on thecurrent flow in said. tubes, the output electrodes of said tubes beingconnected in parallel.

9. In combination, means for forming an electrical image of a field tobe pictured, a shield having an aperture therein substantially in thedimension of said image in length, a target protected by said shieldpositioned to receive the portion of said electrical image passingthrough said aperture, means for deflecting said image across saidaperture in the direction of the length thereof, the rates of deflectionin opposite directions being unequal;

10. In combination, means for forming an electrical image of a field tobe pictured, a shield having an aperture therein substantially in theplane of said image, said aperture being substantially equal to thedimension of said image in length and of varying width such that theoutline of said aperture corresponds to a predetermined wave form andmeans for traversing said the length of the aperture.

1 In combination, means for forming. an

plane of said image, said aperture being of elementary width andsubstantially equal to the aperture across said image in the directionof V electrical image of a field to be pictured, a shield having anaperture, therein substantially in the plane of said image, saidaperture being substantially equal to the dimension of said image inlength and of varying width such that the outline of said aperturecorresponds to the form of a Wave having a predetermined frequencyspectrum.

12. In combination, means for forming. an electrical image of a field tobe pictured a shield having an aperture therein substantially in theplane of said image, said aperture being substantially equal to thedimension of said image in length and of varying width such that theoutline of said aperture corresponds to the form of the I wave resultingfrom the attenuation of a predetermined band of high frequencies from asquarefront wave.

13. In a scanning system, a movable scanning element having linearapertures therein, each aperture being substantially equal in length toa line to be scanned thereby across the picture field, means fortraversing said apertures across said field in the direction of theirlength, and

means for suddenly occulting said apertures.

14. Ina scanning system, a first movable scanning element having linearapertures therein,

. each aperture being substantially equal in length to the line to bescanned thereby across the picture field, and asecond movable scanningelement having serrations formed thereon and so mounted relatively tosaid first element that said serrations suddenly occult said aperturesupon relative motion of both of said elements.

DONALD K. LIPPINCOTT.

